a) Field of the Invention
The present invention relates to an improved drill head for use to drill a hole in a front of hard material such as rock.
The invention more particularly relates to an improved drill head of the type disclosed, by way of examples, in U.S. Pat. Nos. 4,410,053 and 4,840,235 both granted to the present inventor, which drill head is specially designed to drill large vertical holes such as ventilation, escape and/or safety shafts in mines.
b) Brief Description of the Prior Art
To drill large vertical holes in the mining industry, it is of common practice first to drill with a drill shaft, a small pilot hole from the surface down to the gallery where a vertical shaft is to be provided, and then to use the drill shaft to pull upwardly from the gallery a large drill head, while using the pilot hole to guide it.
The large drill head that is used in the second step of the above mentioned method, basically comprises:
a main body acting as a central support; PA1 an upwardly projecting spindle secured on top of the main body, the spindle having a vertical axis and being preferably sized to fit into the pilot hole to guide the drill head; PA1 a fixation head on top of the spindle for detachably connecting it to the drill shaft which is operable in use to rotate the drill head about the vertical axis of the spindle and to pull it up toward the front of hard material to be drilled; and PA1 at least two and preferably three impact hammers mounted onto the main body in such a manner as to fragment in use the hard material of the front along at least one and preferably two circular path(s) centered onto the vertical axis, each impact hammer having a hollow casing, a pneumatically-operated, impact-generating assembly enclosed within the casing and a drill bit actuated by the impact generating assembly. PA1 a main body acting as a central support, this main body having a top end and a bottom end; PA1 a spindle secured to the top end of the mainbody, and upwardly projecting from this mainbody, the spindle having a vertical axis; PA1 means for detachably connecting the spindle to a drill shaft extending in a pilot hole made into a front of hard material to be drilled; the drill shaft being operable in use to rotate the drill head about the vertical axis of the spindle, and to pull it up toward the front of hard material; and PA1 at least two impact hammers mounted onto the main body in such a manner as to fragment in use the hard material of this front along at least one circular path centered onto the vertical axis of the spindle, each of the impact hammers comprising: PA1 a hollow casing defining a central cavity having an upper open end and a lower end, this central cavity having a longitudinal axis substantially parallel vertical axis of the spindle;
The drill heads that are disclosed in the above mentioned U.S. patents are examples of this type of drill head, which distinguish over the other existing drill heads of the same type in that their impact hammers are located so that the inner circular path be spaced apart from the pilot hole and that each outer circular path, if any, be spaced apart from the inner one and from each other, and project at different vertical level. As a result, the hard material is fragmented by impact along each path and simultaneously by shear betwen the pilot hole and the inner path and between each set of adjacent paths, if any, thereby making the drilling operation much faster than with a conventional drill head of the above mentioned type.
If the above mentioned drill heads are quite efficient, their use presents several drawbacks due to their particular construction.
A major one of these drawbacks is the manner in which the hammers are fixed to the main body acting as central support. In U.S. Pat. No. 4,410,053, this is done by means of a pair of pillow-blocks, each involving a half cylindrical seat which is fixed to the central support, by bolting or welding, and into which one end of the cylindrical casing of the hammer rests, this end being clamped in the seat by a metal strap bolted to the seat and forcing the hammer case against it. Under the severe action of the impact hammers in use, the bolts of the pillow-blocks rapidly become loose or the straps or seat break, necessitating break-down of the operation for tightening of the bolts or for repair.
In order to tentatively overcome or at least reduce this major drawback, it has been suggested to use impact hammers each provided with a one-piece cast casing including a connection flange on each side of its longitudinal axis. These flanges which preferably extend the full length of the case, can be bolted to the central support, thereby transmitting the hammer blows directly to the central support and thus appreciably reducing any tendency of the bolts to become loose.
This solution has some advantages, but it calls for a very specific casting which is quite heavy and thus makes the drill head provided with at least two of them very heavy.
Another major drawback of the existing drill heads like the one of U.S. Pat. No. 4,410,053, is that their upper sub-member which receives and guides the drill bit, is merely screwed into the corresponding end of the hammer casing. In operation, this sub-member becomes loose under the severe blows applied to it by the repeated hammering action of the drill bit, rendering break-down adjustment necesary.
In order to tentatively overcome this other major drawbvack and thus avoid loosening of the upper sub-member, it has also been suggested to use impact hammers each comprising a one-piece cast casing formed with radial connection ears at the "sub end" therof. Each hammer also comprises an upper sub-member which has radial connection ears operatively engaging over the casing connection ears while bolt means releasably secure the drive sub-member and casing together through the connection ears.
Once again, this solution has some advantages but it calls again for a very specific casting.